Hair particle system as dynamic paint brush not working?
Use Particle Instance modifier.

I used Cylinder as basic mesh, which was instanced via another object's Hair system.

So, this Instanced mesh will be a Dynamic Paint Brush, set brush option to Mesh Volume or Mesh Volume + Proximity to get it work.



Default Cylinder is vertically oriented, but I gave it 90 degree rotation on X axis in Edit mode (note, that you should do transforms in Edit mode, otherwise transformations will affect whole instanced object). Add some loopcuts to make it easier to bend along hair strands.

Cylinder in Edit mode: